A method and an arrangement for controlling a tank-venting system are disclosed in U.S. Pat. No. 4,683,861. In this method, the pulse-duty factor of the tank-venting valve is so adjusted that the percentage enrichment of the combustion mixture supplied to the engine is of the same magnitude for a given tank-venting mixture in all ranges. It should be noted that it is not only a percentage enrichment which occurs but also a percentage leaning of the mixture when the venting vapor contains more air than what corresponds to the stoichiometric composition. The foregoing means that the tank-venting valve is adjusted in dependence upon the particular actual operating state of the engine so that the volume flow of the venting vapor through the tank-venting valve constitutes a specific percentage of the vapor flow which the engine draws in by suction.
The pregiven percentage is referred to an engine which is driven without disturbances. However, if the engine, for example, draws in leakage air then the pregiven pulse-duty factor for the tank-venting valve no longer reads to the same percentage portion of the venting vapor in the total vapor when different air throughputs through the intake pipe occur; instead, the proportion in each case is now dependent upon the air throughput. This means that for each change of the vapor throughput through the engine for changes in the operating state thereof a change of the air ratio of the mixture drawn in by suction occurs which is caused by the percentage of the venting vapor throughput which is no longer appropriate. This change of the air number must be corrected by a mixture controller for each change of the air throughput.